Nanosecond quantitative analysis during a chemical reaction in nanocrystals

Detailed knowledge of the transition states and kinetics of fast reactions in nanoparticles is desirable for many applications, but challenging to access. Now, insight is obtained in nickel oxide reduction, using single-shot electron pulses in an electron microscope with nanosecond resolution.

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Chemical reactions in nanocrystals are difficult to study. This is not only due to the small size of the reactants but, even more, to the very short reaction times. At high temperatures, it might take only microseconds or even nanoseconds until a reaction front has traversed a nanometer-size crystal. Nevertheless, reaction kinetics in nanocrystals is highly important, e.g., in catalysis where the kinetics of oxidation or reduction of nanoparticles is essential for optimizing the catalytic action under extreme conditions. Reaction constants, activation energies or the presence of short-lived transition states are often unknown or just subject of speculation. To make things even more complicated, many solid-state reactions in nanomaterials are irreversible by nature, so the reaction cannot just be repeated to sum up signal emerging from the small objects. What is needed for such studies is an analytical tool with combined high spatial and high temporal resolution.

Now, a novel approach of analytical electron microscopy, giving access to the elemental composition of nanomaterials with nanometer spatial and nanosecond time resolution, was developed at the Institut de Physique et Chimie des Matériaux in Strasbourg (France). An ultrafast transmission electron microscope (UTEM) that has been designed within an EQUIPEX project of the French national excellence initiative is used to study irreversible chemical reactions with nanometer and nanosecond precision (Fig. 1). In a pump-probe approach, chemical reactions are induced thermally by an intense laser pulse and studied after an adjustable delay with a nanosecond electron pulse in the TEM.

Fig. 1: The ultrafast transmission electron microscope working with short laser and electron pulses.

The technical development has been challenging because the experiments have to be carried out in a single-shot approach. Only one TEM working with pulsed electron beams in the single-shot mode has been demonstrated previously at Lawrence Livermore Laboratories but that microscope wasn't equipped with analytical tools. It has even been speculated that analytical techniques like electron energy-loss spectroscopy (EELS) will never be realizable in the single-shot approach due to the detrimental effects of electron-electron repulsion within the intense electron pulses.

Now, after having developed EELS in the single-shot operation of a TEM, we show that this task is achievable. Although the mutual repulsion of electrons within the pulses still limit spatial, temporal and energy resolution, combined imaging, diffraction and EELS studies turned out to be possible so that a comprehensive picture of the reaction kinetics in nanocrystals can be obtained (Fig. 2). We have demonstrated this technique in a study of the fast reduction of nickel oxide nanocrystals at high temperature. The combination of imaging, electron diffraction and EELS gives unprecedented insight into the dynamics of the reaction. The kinetics of the reaction, in particular the reaction order and rate constant as well as the presence of liquid nickel as a transition state, allows the detailed understanding of the reaction mechanisms. With this new technique of fast analysis, the reaction kinetics in small solid systems can now be studied in detail. The present experiments have been done in vacuum but are also possible in a gas atmosphere when environmental TEM stages are used. The new approach has the potential to replace the complicated combination of different characterization techniques such as diffraction in a synchrotron, optical spectroscopy and microscopy.

Fig. 2: An infrared laser pulse induces the reduction of NiO into Ni nanocrystals. Taking electron energy-loss spectra (EELS) with nanosecond electron pulses at different delays after the laser pulse allows the quantitative elemental analysis during the fast reaction.

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